The olefin metathesis process can be defined as the redistribution of alkylidene moieties to give a mixture of olefins, e.g., propylene is converted to ethylene and butylene. The simplest example is: EQU 2R'CH.dbd.CHR.revreaction.RCH.dbd.CHR+R'CH.dbd.CHR'
The reaction proceeds by addition of an olefin to a catalyst having a metal-carbon double bond ([M].dbd.CHR; metal alkylidene complex) to give a metallacyclobutane ring, which then releases an olefin to reform a metal-alkylidene complex. A typical olefin of interest that will undergo metathesis in the presence of an appropriate catalyst is an ester of oleic acid, cis-CH.sub.3 (CH.sub.2).sub.7 CH.dbd.CH(CH.sub.2).sub.7 CO.sub.2 H.
Ring-opening metathesis polymerization (ROMP) of cycloolefins by olefin metathesis catalysts is an extremely important facet of the olefin metathesis reaction that results in the preparation of thermoset and thermoplastic polymers. Monocyclic olefins, substituted monocyclics, polycyclic and substituted polycyclic olefins have all been employed in the preparation of unsaturated polymers. In many cases, cycloolefins can be polymerized even when they contain polar or reactive functional groups. Norbornene is a typical olefin that will undergo ROMP in the presence of an olefin metathesis catalyst having a metal-carbon double bond.
A third type of olefin metathesis reaction is acyclic diene metathesis (ADMET). ADMET chemistry can be used to form dimers, oligomers, or polymers. ADMET polymerization is a viable synthetic route to high molecular weight polymers and copolymers, but it is an inherently more complicated process than ring-opening metathesis polymerization (ROMP). ROMP is a chain growth, addition-type polymerization driven by the alleviation of ring strain. The ADMET reaction is a step propagation condensation reaction. The ADMET reaction is an equilibrium process wherein the product is generated by the removal of an olefin, typically ethylene, from the reaction. Thus, in the ADMET (or simple olefin metathesis) reaction of styrene (a monoolefin), quantitative metathesis chemistry occurs yielding the expected dimer of trans-stilbene. At high conversion, ADMET polymerization of 1,9-decadiene (an .alpha., .omega.-diolefin) yields poly(octylene). When ADMET polymerization reactions are terminated before reaching .apprxeq.99% conversion, the .alpha., .omega.-diolefins employed generate oligomers, since step growth polymerization only produces high molecular weight polymer at very high conversions. An important aspect of ADMET polymerization is that ester and ether functionalities are often tolerated on the time scale of the polymerization reaction, and polymers can be prepared that are not directly available in a ROMP reaction.
Because the ADMET reaction is a step growth equilibrium condensation reaction, it provides the opportunity to shift the equilibrium between monomer and polymer. It has been found that unsaturated polymers, such as polynorbornene and polybutadiene, can be depolymerized in combination with ethylene to yield low molecular weight oligomers and .alpha., .omega.-alkadienes.
The olefin metathesis reaction has also been employed in ring-closing metathesis of acyclic diene ethers to form cyclic and acyclic olefins (Fu and Grubbs, J. Am. Chem. Soc. 1992, Vol. 114, p. 5426-27). For example, various diallyl ethers when treated with a well-defined alkylidene complex, Mo(CHCMe.sub.2 Ph) (NC.sub.6 H.sub.3 -2,6-i-Pr.sub.2) (OCMe(CF.sub.3).sub.2).sub.2, lead to a number of ring-closed derivatives, i.e., cyclic ethers, amines and amides.
To reap the benefits of functionalized olefin metathesis, ring-opening metathesis, ring-closure chemistry, and the ADMET chemistry described above, the trend has been towards the formation of less Lewis acidic, well-characterized catalyst species based on a variety of transition metals including tungsten and molybdenum. These catalysts should be simple to prepare, of known structure, and with a reactivity that can be controlled. Several of these types of compounds have been shown to metathesize olefins with an activity that can be regulated through the choice of aryloxide or alkoxide ligands. For examples, see J. M. Basset et al., Angew. Chem., 1992, Vol. 104, p. 622-664; J. A. Osborn et al., J. Chem. Soc., Chem. Commun., 1989, p. 1062-1063; R. R. Schrock, U.S. Pat. Nos. 4,681,956 and 4,727,215, and R. R. Schrock et al., U. S. Pat. No. 5,142,073.
Tungsten and molybdenum catalysts, M(.dbd.CHR.sup.3) (NR.sup.l) (OR.sup.2).sub.2, reported by R. R. Schrock have been employed in all of the areas of olefin metathesis described above. These well-defined transition metal alkylidene complexes have a high tolerance for functionalized groups as well as temperature. Molybdenum and tungsten alkylidene complexes of the following compositions are particularly useful: Mo(CHCMe.sub.2 Ph) (NC.sub.6 H.sub.3 -2, 6-i-Pr.sub.2) (OCMe(CF.sub.3).sub.2).sub.2, Mo(CHCMe.sub.3) (NC.sub.6 H.sub.3 2,6-i-Pr.sub.2) (OCMe.sub.3).sub.2, Mo(CHCMe.sub.3) (NC.sub.6 H.sub.3 -2,6-i-Pr.sub.2) (OCMe(CF.sub.3).sub.2).sub.2, W(NC.sub.6 H.sub.3 -2,6-i-Pr.sub.2) (CHCMe.sub.2 Ph) (OCMe.sub.3).sub.2, W(NC.sub.6 H.sub.3 -2,6-i-Pr.sub.2) (CHCMe.sub.3) (OCMe.sub.3).sub.2, W(NC.sub.6 H.sub.3 -2,6-i-Pr.sub.2) (CHCMe.sub.2 Ph) (OCMe(CF.sub.3).sub.2).sub.2 W(NC.sub.6 H.sub.3 -2,6-i-Pr.sub.2) (CHCMe.sub.3) (OCMe(CF.sub.3).sub.2).sub.2, W(NC.sub.6 H.sub.3 -2,6-i-Pr.sub.2) (CHCMe.sub.2 Ph).sub.2 (OCMe(CF.sub.3).sub.2).sub.2, and W(NC.sub.6 H.sub.3 -2,6-i-Pr.sub.2) (CHCMe.sub.3 Ph).sub.2 (OC(CF.sub.3) (CF.sub.2 CF.sub.3).sub.2. These catalysts, however, are typically prepared using multistep synthetic routes that are quite labor intensive and often use expensive reagents.
The simple olefin metathesis reaction and ring-opening metathesis polymerization synthesis of specialty polymers using these catalysts are performed in homogeneous phase by reacting the olefin with the alkylidene species in a solvent. The ADMET reaction employing these catalysts can be performed both in solution and in bulk monomer.
In the manufacture of thermoset polymers, it is desirable to have a solventless system so that solvent is not trapped within the polymer matrix. One disadvantage to the use of well-defined alkylidene catalysts is that they initiate polymerization (or olefin metathesis) immediately upon contact with a metathesizable monomer.
Reaction injection molding (RIM) of polyolefins by ring-opening of polyolefin metathesis polymerizable monomers in the presence of alkylidene complexes is a particularly important aspect of olefin metathesis. For example, Klosiewicz (U.S. Pat. Nos. 4,400,340 and 4,520,181) discusses a method whereby polydicyclopentadiene can be prepared by combining a plurality of reactant streams. One stream contains a metathesis catalyst and the second stream the activator for the metathesis system, and at least one contains dicyclopentadiene. After mixing the streams and generating a metathesis catalyst, the mixture is immediately injected into a mold where polymerization takes place. Although such catalysts are very effective species for polymerization, residual amounts of chlorine or solvent for the dissolution of the catalyst precursor or activator are retained in the thermoset product.